1. Field of the Invention
The invention relates to a method of deriving a spin resonance signal from a moving fluid which is subjected to a magnetic field consisting of a constant field component, one or more gradient field components, at least one of which extends in the direction of motion of the fluid, and a magnetic field component which is generated by r.f. electromagnetic signals perpendicularly to the constant field component in order to excite the nuclear spins of the fluid. The invention also relates to a device for performing the method.
2. Description of Related Art
A method of this kind is known inter alia from "Flow Imaging by Nuclear Magnetic Resonance", Van As et al, "Annales de Radiologie", Vol. 27, No. 5, 1984, pp. 405-413, and "The Study of Flow by Pulsed Nuclear Magnetic Resonance. II Measurement of Flow Velocities Using a Repetitive Pulse Method", Hemminga M. A. and de Jager P. A., "Journal of Magnetic Resonance", No. 37, 1980, pp. 1-16.
The state of the art method enables non-invasive determination of the velocity of a continuously or non-continuously moving fluid, possibly in the presence of an excess of stationary fluid in the immediate vicinity of the moving fluid. In this context a moving fluid is to be understood to mean a fluid flowing in an object, a stationary fluid in a moving object, or combinations of both.
According to the known method, the object in which the fluid is present is arranged between the poles of a magnet and is enclosed by a wire coil whereby the fluid is subjected to a series of brief r.f. electromagnetic pulses of equal duration. Moreover, a magnetic field gradient which extends in the direction of motion is applied.
In the state of equilibrium, in the absence of a magnetic field component generated by the r.f. electromagnetic pulses, the individual nuclear spins in the fluid perform a precessional motion around the constant magnetic field component. Using the r.f. magnetic field component, the nuclear spins can be excited so that they are rotated with respect to the constant field component. During the interval between the successive r.f. pulses the nuclear spins fan out and tend to resume the direction of the constant field component. Inter alia the linear and the volumetric flow rate of the fluid can be determined from the shape of the electric signals then generated in the wire coil. After calibration, each of said quantities can be measured in an absolute sense.
This known method has the drawback that the calibration curves for the relationship between the flow quantities and the measured signal depend greatly on the flow profile, and also that these calibration curves are determined to a high degree by the spin-spin and spin-lattice relaxation times of the fluid to be measured. The spin-spin relaxation time is a measure of the speed at which the nuclear spins fan out with respect to one another, the spin lattice relaxation time being a measure of the speed at which the nuclear spins return in the direction of the constant field component. In biological materials, notably the spin-spin relaxation time may be dependent on a large number of factors and may vary strongly, thus introducing a high degree of uncertainty in the interpretation of the measured signals so that comparatively large measurement errors may arise.
The effect of the spin-spin relaxation time on the measurement results can essentially be considered as the effect of a low-pass filter. In the case of a fluid whose flow varies in time and/or in an object whose motion varies in time, this filter effect distorts the measured signal, so that reliable and accurate measurements of the spin-spin relaxation time and the flow properties are not possible. In the case of imaging by means of the nuclear spin resonance technique, the spin-spin relaxation time has an adverse effect on the resolution of the images formed.